=head1 NAME
-perlfaq5 - Files and Formats ($Revision: 1.19 $)
+perlfaq5 - Files and Formats ($Revision: 1.38 $, $Date: 1999/05/23 16:08:30 $)
=head1 DESCRIPTION
This section deals with I/O and the "f" issues: filehandles, flushing,
formats, and footers.
-=head2 How do I flush/unbuffer a filehandle? Why must I do this?
+=head2 How do I flush/unbuffer an output filehandle? Why must I do this?
The C standard I/O library (stdio) normally buffers characters sent to
devices. This is done for efficiency reasons, so that there isn't a
Perl, you go though this buffering. syswrite() circumvents stdio and
buffering.
-In most stdio implementations, the type of buffering and the size of
+In most stdio implementations, the type of output buffering and the size of
the buffer varies according to the type of device. Disk files are block
buffered, often with a buffer size of more than 2k. Pipes and sockets
are often buffered with a buffer size between 1/2 and 2k. Serial devices
get the output where you want it when you want it.
If you expect characters to get to your device when you print them there,
-you'll want to autoflush its handle, as in the older:
+you'll want to autoflush its handle.
+Use select() and the C<$|> variable to control autoflushing
+(see L<perlvar/$|> and L<perlfunc/select>):
+
+ $old_fh = select(OUTPUT_HANDLE);
+ $| = 1;
+ select($old_fh);
+
+Or using the traditional idiom:
+
+ select((select(OUTPUT_HANDLE), $| = 1)[0]);
+
+Or if don't mind slowly loading several thousand lines of module code
+just because you're afraid of the C<$|> variable:
use FileHandle;
- open(DEV, "<+/dev/tty"); # ceci n'est pas une pipe
+ open(DEV, "+</dev/tty"); # ceci n'est pas une pipe
DEV->autoflush(1);
or the newer IO::* modules:
die "$!" unless $sock;
$sock->autoflush();
- $sock->print("GET /\015\012");
- $document = join('', $sock->getlines());
+ print $sock "GET / HTTP/1.0" . "\015\012" x 2;
+ $document = join('', <$sock>);
print "DOC IS: $document\n";
-Note the hardcoded carriage return and newline in their octal
-equivalents. This is the ONLY way (currently) to assure a proper
-flush on all platforms, including Macintosh.
-
-You can use select() and the C<$|> variable to control autoflushing
-(see L<perlvar/$|> and L<perlfunc/select>):
-
- $oldh = select(DEV);
- $| = 1;
- select($oldh);
+Note the bizarrely hardcoded carriage return and newline in their octal
+equivalents. This is the ONLY way (currently) to assure a proper flush
+on all platforms, including Macintosh. That's the way things work in
+network programming: you really should specify the exact bit pattern
+on the network line terminator. In practice, C<"\n\n"> often works,
+but this is not portable.
-You'll also see code that does this without a temporary variable, as in
-
- select((select(DEV), $| = 1)[0]);
+See L<perlfaq9> for other examples of fetching URLs over the web.
=head2 How do I change one line in a file/delete a line in a file/insert a line in the middle of a file/append to the beginning of a file?
+Those are operations of a text editor. Perl is not a text editor.
+Perl is a programming language. You have to decompose the problem into
+low-level calls to read, write, open, close, and seek.
+
Although humans have an easy time thinking of a text file as being a
-sequence of lines that operates much like a stack of playing cards --
-or punch cards -- computers usually see the text file as a sequence of
-bytes. In general, there's no direct way for Perl to seek to a
-particular line of a file, insert text into a file, or remove text
-from a file.
+sequence of lines that operates much like a stack of playing cards -- or
+punch cards -- computers usually see the text file as a sequence of bytes.
+In general, there's no direct way for Perl to seek to a particular line
+of a file, insert text into a file, or remove text from a file.
-(There are exceptions in special circumstances. Replacing a sequence
-of bytes with another sequence of the same length is one. Another is
-using the C<$DB_RECNO> array bindings as documented in L<DB_File>.
-Yet another is manipulating files with all lines the same length.)
+(There are exceptions in special circumstances. You can add or remove at
+the very end of the file. Another is replacing a sequence of bytes with
+another sequence of the same length. Another is using the C<$DB_RECNO>
+array bindings as documented in L<DB_File>. Yet another is manipulating
+files with all lines the same length.)
The general solution is to create a temporary copy of the text file with
-the changes you want, then copy that over the original.
+the changes you want, then copy that over the original. This assumes
+no locking.
$old = $file;
$new = "$file.tmp.$$";
- $bak = "$file.bak";
+ $bak = "$file.orig";
open(OLD, "< $old") or die "can't open $old: $!";
open(NEW, "> $new") or die "can't open $new: $!";
perl -pi -e 's/(^\s+test\s+)\d+/ $1 . ++$count /e' t/op/taint.t
# form a script
- local($^I, @ARGV) = ('.bak', glob("*.c"));
+ local($^I, @ARGV) = ('.orig', glob("*.c"));
while (<>) {
if ($. == 1) {
print "This line should appear at the top of each file\n";
whole file into memory:
open (FH, "+< $file");
- while ( <FH> ) { $addr = tell(FH) unless eof(FH) }
+ while ( <FH> ) { $addr = tell(FH) unless eof(FH) }
truncate(FH, $addr);
Error checking is left as an exercise for the reader.
}
close FILE;
+This assumes no funny games with newline translations.
+
=head2 How do I make a temporary file name?
-Use the process ID and/or the current time-value. If you need to have
-many temporary files in one process, use a counter:
+Use the C<new_tmpfile> class method from the IO::File module to get a
+filehandle opened for reading and writing. Use this if you don't
+need to know the file's name.
+
+ use IO::File;
+ $fh = IO::File->new_tmpfile()
+ or die "Unable to make new temporary file: $!";
+
+Or you can use the C<tmpnam> function from the POSIX module to get a
+filename that you then open yourself. Use this if you do need to know
+the file's name.
+
+ use Fcntl;
+ use POSIX qw(tmpnam);
+
+ # try new temporary filenames until we get one that didn't already
+ # exist; the check should be unnecessary, but you can't be too careful
+ do { $name = tmpnam() }
+ until sysopen(FH, $name, O_RDWR|O_CREAT|O_EXCL);
+
+ # install atexit-style handler so that when we exit or die,
+ # we automatically delete this temporary file
+ END { unlink($name) or die "Couldn't unlink $name : $!" }
+
+ # now go on to use the file ...
+
+If you're committed to doing this by hand, use the process ID and/or
+the current time-value. If you need to have many temporary files in
+one process, use a counter:
BEGIN {
- use IO::File;
use Fcntl;
my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMP} || $ENV{TEMP};
my $base_name = sprintf("%s/%d-%d-0000", $temp_dir, $$, time());
sub temp_file {
- my $fh = undef;
+ local *FH;
my $count = 0;
- until (defined($fh) || $count > 100) {
+ until (defined(fileno(FH)) || $count++ > 100) {
$base_name =~ s/-(\d+)$/"-" . (1 + $1)/e;
- $fh = IO::File->new($base_name, O_WRONLY|O_EXCL|O_CREAT, 0644)
+ sysopen(FH, $base_name, O_WRONLY|O_EXCL|O_CREAT);
}
- if (defined($fh)) {
- return ($fh, $base_name);
+ if (defined(fileno(FH))
+ return (*FH, $base_name);
} else {
return ();
}
}
}
-Or you could simply use IO::Handle::new_tmpfile.
-
=head2 How can I manipulate fixed-record-length files?
-The most efficient way is using pack() and unpack(). This is faster
-than using substr(). Here is a sample chunk of code to break up and
-put back together again some fixed-format input lines, in this case
-from the output of a normal, Berkeley-style ps:
+The most efficient way is using pack() and unpack(). This is faster than
+using substr() when taking many, many strings. It is slower for just a few.
+
+Here is a sample chunk of code to break up and put back together again
+some fixed-format input lines, in this case from the output of a normal,
+Berkeley-style ps:
# sample input line:
# 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what
$PS_T = 'A6 A4 A7 A5 A*';
open(PS, "ps|");
- $_ = <PS>; print;
+ print scalar <PS>;
while (<PS>) {
($pid, $tt, $stat, $time, $command) = unpack($PS_T, $_);
for $var (qw!pid tt stat time command!) {
"\n";
}
+We've used C<$$var> in a way that forbidden by C<use strict 'refs'>.
+That is, we've promoted a string to a scalar variable reference using
+symbolic references. This is ok in small programs, but doesn't scale
+well. It also only works on global variables, not lexicals.
+
=head2 How can I make a filehandle local to a subroutine? How do I pass filehandles between subroutines? How do I make an array of filehandles?
-You may have some success with typeglobs, as we always had to use
-in days of old:
+The fastest, simplest, and most direct way is to localize the typeglob
+of the filehandle in question:
- local(*FH);
+ local *TmpHandle;
-But while still supported, that isn't the best to go about getting
-local filehandles. Typeglobs have their drawbacks. You may well want
-to use the C<FileHandle> module, which creates new filehandles for you
-(see L<FileHandle>):
+Typeglobs are fast (especially compared with the alternatives) and
+reasonably easy to use, but they also have one subtle drawback. If you
+had, for example, a function named TmpHandle(), or a variable named
+%TmpHandle, you just hid it from yourself.
- use FileHandle;
sub findme {
- my $fh = FileHandle->new();
- open($fh, "</etc/hosts") or die "no /etc/hosts: $!";
- while (<$fh>) {
+ local *HostFile;
+ open(HostFile, "</etc/hosts") or die "no /etc/hosts: $!";
+ local $_; # <- VERY IMPORTANT
+ while (<HostFile>) {
print if /\b127\.(0\.0\.)?1\b/;
}
- # $fh automatically closes/disappears here
+ # *HostFile automatically closes/disappears here
+ }
+
+Here's how to use this in a loop to open and store a bunch of
+filehandles. We'll use as values of the hash an ordered
+pair to make it easy to sort the hash in insertion order.
+
+ @names = qw(motd termcap passwd hosts);
+ my $i = 0;
+ foreach $filename (@names) {
+ local *FH;
+ open(FH, "/etc/$filename") || die "$filename: $!";
+ $file{$filename} = [ $i++, *FH ];
+ }
+
+ # Using the filehandles in the array
+ foreach $name (sort { $file{$a}[0] <=> $file{$b}[0] } keys %file) {
+ my $fh = $file{$name}[1];
+ my $line = <$fh>;
+ print "$name $. $line";
+ }
+
+For passing filehandles to functions, the easiest way is to
+preface them with a star, as in func(*STDIN). See L<perlfaq7/"Passing
+Filehandles"> for details.
+
+If you want to create many anonymous handles, you should check out the
+Symbol, FileHandle, or IO::Handle (etc.) modules. Here's the equivalent
+code with Symbol::gensym, which is reasonably light-weight:
+
+ foreach $filename (@names) {
+ use Symbol;
+ my $fh = gensym();
+ open($fh, "/etc/$filename") || die "open /etc/$filename: $!";
+ $file{$filename} = [ $i++, $fh ];
+ }
+
+Or here using the semi-object-oriented FileHandle module, which certainly
+isn't light-weight:
+
+ use FileHandle;
+
+ foreach $filename (@names) {
+ my $fh = FileHandle->new("/etc/$filename") or die "$filename: $!";
+ $file{$filename} = [ $i++, $fh ];
}
-Internally, Perl believes filehandles to be of class IO::Handle. You
-may use that module directly if you'd like (see L<IO::Handle>), or
-one of its more specific derived classes.
+Please understand that whether the filehandle happens to be a (probably
+localized) typeglob or an anonymous handle from one of the modules,
+in no way affects the bizarre rules for managing indirect handles.
+See the next question.
+
+=head2 How can I use a filehandle indirectly?
+
+An indirect filehandle is using something other than a symbol
+in a place that a filehandle is expected. Here are ways
+to get those:
+
+ $fh = SOME_FH; # bareword is strict-subs hostile
+ $fh = "SOME_FH"; # strict-refs hostile; same package only
+ $fh = *SOME_FH; # typeglob
+ $fh = \*SOME_FH; # ref to typeglob (bless-able)
+ $fh = *SOME_FH{IO}; # blessed IO::Handle from *SOME_FH typeglob
+
+Or to use the C<new> method from the FileHandle or IO modules to
+create an anonymous filehandle, store that in a scalar variable,
+and use it as though it were a normal filehandle.
+
+ use FileHandle;
+ $fh = FileHandle->new();
+
+ use IO::Handle; # 5.004 or higher
+ $fh = IO::Handle->new();
+
+Then use any of those as you would a normal filehandle. Anywhere that
+Perl is expecting a filehandle, an indirect filehandle may be used
+instead. An indirect filehandle is just a scalar variable that contains
+a filehandle. Functions like C<print>, C<open>, C<seek>, or
+the C<< <FH> >> diamond operator will accept either a read filehandle
+or a scalar variable containing one:
+
+ ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR);
+ print $ofh "Type it: ";
+ $got = <$ifh>
+ print $efh "What was that: $got";
+
+If you're passing a filehandle to a function, you can write
+the function in two ways:
+
+ sub accept_fh {
+ my $fh = shift;
+ print $fh "Sending to indirect filehandle\n";
+ }
+
+Or it can localize a typeglob and use the filehandle directly:
+
+ sub accept_fh {
+ local *FH = shift;
+ print FH "Sending to localized filehandle\n";
+ }
+
+Both styles work with either objects or typeglobs of real filehandles.
+(They might also work with strings under some circumstances, but this
+is risky.)
+
+ accept_fh(*STDOUT);
+ accept_fh($handle);
+
+In the examples above, we assigned the filehandle to a scalar variable
+before using it. That is because only simple scalar variables,
+not expressions or subscripts into hashes or arrays, can be used with
+built-ins like C<print>, C<printf>, or the diamond operator. These are
+illegal and won't even compile:
+
+ @fd = (*STDIN, *STDOUT, *STDERR);
+ print $fd[1] "Type it: "; # WRONG
+ $got = <$fd[0]> # WRONG
+ print $fd[2] "What was that: $got"; # WRONG
+
+With C<print> and C<printf>, you get around this by using a block and
+an expression where you would place the filehandle:
+
+ print { $fd[1] } "funny stuff\n";
+ printf { $fd[1] } "Pity the poor %x.\n", 3_735_928_559;
+ # Pity the poor deadbeef.
+
+That block is a proper block like any other, so you can put more
+complicated code there. This sends the message out to one of two places:
+
+ $ok = -x "/bin/cat";
+ print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\n";
+ print { $fd[ 1+ ($ok || 0) ] } "cat stat $ok\n";
+
+This approach of treating C<print> and C<printf> like object methods
+calls doesn't work for the diamond operator. That's because it's a
+real operator, not just a function with a comma-less argument. Assuming
+you've been storing typeglobs in your structure as we did above, you
+can use the built-in function named C<readline> to reads a record just
+as C<< <> >> does. Given the initialization shown above for @fd, this
+would work, but only because readline() require a typeglob. It doesn't
+work with objects or strings, which might be a bug we haven't fixed yet.
+
+ $got = readline($fd[0]);
+
+Let it be noted that the flakiness of indirect filehandles is not
+related to whether they're strings, typeglobs, objects, or anything else.
+It's the syntax of the fundamental operators. Playing the object
+game doesn't help you at all here.
=head2 How can I set up a footer format to be used with write()?
-There's no built
-in way to do this, but L<perlform> has a couple of
+There's no builtin way to do this, but L<perlform> has a couple of
techniques to make it possible for the intrepid hacker.
=head2 How can I write() into a string?
-See L<perlform> for an swrite() function.
+See L<perlform/"Accessing Formatting Internals"> for an swrite() function.
=head2 How can I output my numbers with commas added?
sub commify {
local $_ = shift;
- 1 while s/^(-?\d+)(\d{3})/$1,$2/;
+ 1 while s/^([-+]?\d+)(\d{3})/$1,$2/;
return $_;
}
You can't just:
- s/^(-?\d+)(\d{3})/$1,$2/g;
+ s/^([-+]?\d+)(\d{3})/$1,$2/g;
because you have to put the comma in and then recalculate your
position.
+Alternatively, this commifies all numbers in a line regardless of
+whether they have decimal portions, are preceded by + or -, or
+whatever:
+
+ # from Andrew Johnson <ajohnson@gpu.srv.ualberta.ca>
+ sub commify {
+ my $input = shift;
+ $input = reverse $input;
+ $input =~ s<(\d\d\d)(?=\d)(?!\d*\.)><$1,>g;
+ return scalar reverse $input;
+ }
+
=head2 How can I translate tildes (~) in a filename?
-Use the
E<lt>E<gt> (glob()) operator, documented in L<perlfunc>. This
+Use the
<> (glob()) operator, documented in L<perlfunc>. This
requires that you have a shell installed that groks tildes, meaning
csh or tcsh or (some versions of) ksh, and thus may have portability
problems. The Glob::KGlob module (available from CPAN) gives more
: ( $ENV{HOME} || $ENV{LOGDIR} )
}ex;
-=head2 How come when I open the file read-write it wipes it out?
+=head2 How come when I open a file read-write it wipes it out?
Because you're using something like this, which truncates the file and
I<then> gives you read-write access:
- open(FH, "+> /path/name"); # WRONG
+ open(FH, "+> /path/name"); # WRONG (almost always)
Whoops. You should instead use this, which will fail if the file
-doesn't exist.
+doesn't exist.
- open(FH, "+< /path/name"); # open for update
+ open(FH, "+< /path/name"); # open for update
-If this is an issue, try:
+Using ">" always clobbers or creates. Using "<" never does
+either. The "+" doesn't change this.
- sysopen(FH, "/path/name", O_RDWR|O_CREAT, 0644);
+Here are examples of many kinds of file opens. Those using sysopen()
+all assume
-Error checking is left as an exercise for the reader.
+ use Fcntl;
+
+To open file for reading:
+
+ open(FH, "< $path") || die $!;
+ sysopen(FH, $path, O_RDONLY) || die $!;
+
+To open file for writing, create new file if needed or else truncate old file:
+
+ open(FH, "> $path") || die $!;
+ sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT) || die $!;
+ sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666) || die $!;
+
+To open file for writing, create new file, file must not exist:
+
+ sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT) || die $!;
+ sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0666) || die $!;
+
+To open file for appending, create if necessary:
+
+ open(FH, ">> $path") || die $!;
+ sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT) || die $!;
+ sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT, 0666) || die $!;
+
+To open file for appending, file must exist:
+
+ sysopen(FH, $path, O_WRONLY|O_APPEND) || die $!;
+
+To open file for update, file must exist:
+
+ open(FH, "+< $path") || die $!;
+ sysopen(FH, $path, O_RDWR) || die $!;
+
+To open file for update, create file if necessary:
+
+ sysopen(FH, $path, O_RDWR|O_CREAT) || die $!;
+ sysopen(FH, $path, O_RDWR|O_CREAT, 0666) || die $!;
+
+To open file for update, file must not exist:
+
+ sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT) || die $!;
+ sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT, 0666) || die $!;
+
+To open a file without blocking, creating if necessary:
+
+ sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT)
+ or die "can't open /tmp/somefile: $!":
+
+Be warned that neither creation nor deletion of files is guaranteed to
+be an atomic operation over NFS. That is, two processes might both
+successful create or unlink the same file! Therefore O_EXCL
+isn't so exclusive as you might wish.
+
+See also the new L<perlopentut> if you have it (new for 5.6).
=head2 Why do I sometimes get an "Argument list too long" when I use <*>?
-The C<E<lt>E<gt>> operator performs a globbing operation (see above).
-By default glob() forks csh(1) to do the actual glob expansion, but
+The C<< <> >> operator performs a globbing operation (see above).
+In Perl versions earlier than v5.6.0, the internal glob() operator forks
+csh(1) to do the actual glob expansion, but
csh can't handle more than 127 items and so gives the error message
C<Argument list too long>. People who installed tcsh as csh won't
have this problem, but their users may be surprised by it.
-To get around this, either do the glob yourself with C<Dirhandle>s and
-patterns, or use a module like Glob::KGlob, one that doesn't use the
-shell to do globbing.
+To get around this, either upgrade to Perl v5.6.0 or later, do the glob
+yourself with readdir() and patterns, or use a module like Glob::KGlob,
+one that doesn't use the shell to do globbing.
=head2 Is there a leak/bug in glob()?
context, you may cause a leak and/or unpredictable behavior. It's
best therefore to use glob() only in list context.
-=head2 How can I open a file with a leading "E<gt>" or trailing blanks?
+=head2 How can I open a file with a leading ">" or trailing blanks?
Normally perl ignores trailing blanks in filenames, and interprets
certain leading characters (or a trailing "|") to mean something
sub safe_filename {
local $_ = shift;
- return m#^/#
- ? "$_\0"
- : "./$_\0";
+ s#^([^./])#./$1#;
+ $_ .= "\0";
+ return $_;
}
- $fn = safe_filename("<<<something really wicked ");
- open(FH, "> $fn") or "couldn't open $fn: $!";
+ $badpath = "<<<something really wicked ";
+ $fn = safe_filename($badpath");
+ open(FH, "> $fn") or "couldn't open $badpath: $!";
+
+This assumes that you are using POSIX (portable operating systems
+interface) paths. If you are on a closed, non-portable, proprietary
+system, you may have to adjust the C<"./"> above.
+
+It would be a lot clearer to use sysopen(), though:
+
+ use Fcntl;
+ $badpath = "<<<something really wicked ";
+ open (FH, $badpath, O_WRONLY | O_CREAT | O_TRUNC)
+ or die "can't open $badpath: $!";
-You could also use the sysopen() function (see L<perlfunc/sysopen>).
+For more information, see also the new L<perlopentut> if you have it
+(new for 5.6).
=head2 How can I reliably rename a file?
-Well, usually you just use Perl's rename() function. But that may
-not work everywhere, in particular, renaming files across file systems.
-If your operating system supports a mv(1) program or its moral equivalent,
-this works:
+Well, usually you just use Perl's rename() function. But that may not
+work everywhere, in particular, renaming files across file systems.
+Some sub-Unix systems have broken ports that corrupt the semantics of
+rename() -- for example, WinNT does this right, but Win95 and Win98
+are broken. (The last two parts are not surprising, but the first is. :-)
+
+If your operating system supports a proper mv(1) program or its moral
+equivalent, this works:
rename($old, $new) or system("mv", $old, $new);
real rename(), though, which preserves metainformation like
permissions, timestamps, inode info, etc.
+The newer version of File::Copy exports a move() function.
+
=head2 How can I lock a file?
-Perl's built
-in flock() function (see L<perlfunc> for details) will call
+Perl's builtin flock() function (see L<perlfunc> for details) will call
flock(2) if that exists, fcntl(2) if it doesn't (on perl version 5.004 and
later), and lockf(3) if neither of the two previous system calls exists.
On some systems, it may even use a different form of native locking.
=item 3
-Some versions of flock() can't lock files over a network (e.g. on NFS
-file systems), so you'd need to force the use of fcntl(2) when you
-build Perl. See the flock entry of L<perlfunc>, and the F<INSTALL>
-file in the source distribution for information on building Perl to do
-this.
+Some versions of flock() can't lock files over a network (e.g. on NFS file
+systems), so you'd need to force the use of fcntl(2) when you build Perl.
+But even this is dubious at best. See the flock entry of L<perlfunc>,
+and the F<INSTALL> file in the source distribution for information on
+building Perl to do this.
+
+Two potentially non-obvious but traditional flock semantics are that
+it waits indefinitely until the lock is granted, and that its locks
+I<merely advisory>. Such discretionary locks are more flexible, but
+offer fewer guarantees. This means that files locked with flock() may
+be modified by programs that do not also use flock(). Cars that stop
+for red lights get on well with each other, but not with cars that don't
+stop for red lights. See the perlport manpage, your port's specific
+documentation, or your system-specific local manpages for details. It's
+best to assume traditional behavior if you're writing portable programs.
+(But if you're not, you should as always feel perfectly free to write
+for your own system's idiosyncrasies (sometimes called "features").
+Slavish adherence to portability concerns shouldn't get in the way of
+your getting your job done.)
+
+For more information on file locking, see also L<perlopentut/"File
+Locking"> if you have it (new for 5.6).
=back
-The CPAN module File::Lock offers similar functionality and (if you
-have dynamic loading) won't require you to rebuild perl if your
-flock() can't lock network files.
-
-=head2 What can't I just open(FH, ">file.lock")?
+=head2 Why can't I just open(FH, ">file.lock")?
A common bit of code B<NOT TO USE> is this:
which must be done in one. That's why computer hardware provides an
atomic test-and-set instruction. In theory, this "ought" to work:
- sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT, 0644)
+ sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT)
or die "can't open file.lock: $!":
except that lamentably, file creation (and deletion) is not atomic
over NFS, so this won't work (at least, not every time) over the net.
-Various schemes involving involving link() have been suggested, but
+Various schemes involving link() have been suggested, but
these tend to involve busy-wait, which is also subdesirable.
-=head2 I still don't get locking. I just want to increment the number
-in the file. How can I do this?
+=head2 I still don't get locking. I just want to increment the number in the file. How can I do this?
Didn't anyone ever tell you web-page hit counters were useless?
+They don't count number of hits, they're a waste of time, and they serve
+only to stroke the writer's vanity. Better to pick a random number.
+It's more realistic.
-Anyway, this is what to do:
+Anyway, this is what you can do if you can't help yourself.
- use Fcntl;
- sysopen(FH, "numfile", O_RDWR|O_CREAT, 0644) or die "can't open numfile: $!";
- flock(FH, 2) or die "can't flock numfile: $!";
+ use Fcntl qw(:DEFAULT :flock);
+ sysopen(FH, "numfile", O_RDWR|O_CREAT) or die "can't open numfile: $!";
+ flock(FH, LOCK_EX) or die "can't flock numfile: $!";
$num = <FH> || 0;
seek(FH, 0, 0) or die "can't rewind numfile: $!";
truncate(FH, 0) or die "can't truncate numfile: $!";
(print FH $num+1, "\n") or die "can't write numfile: $!";
- # DO NOT UNLOCK THIS UNTIL YOU CLOSE
+ # Perl as of 5.004 automatically flushes before unlocking
+ flock(FH, LOCK_UN) or die "can't flock numfile: $!";
close FH or die "can't close numfile: $!";
Here's a much better web-page hit counter:
seek(FH, $recno * $RECSIZE, 0);
read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!";
# munge the record
- seek(FH, $recno * $RECSIZE, 0);
+ seek(FH, -$RECSIZE, 1);
print FH $record;
close FH;
Locking and error checking are left as an exercise for the reader.
Don't forget them, or you'll be quite sorry.
-Don't forget to set binmode() under DOS-like platforms when operating
-on files that have anything other than straight text in them. See the
-docs on open() and on binmode() for more details.
-
=head2 How do I get a file's timestamp in perl?
If you want to retrieve the time at which the file was last read,
Here's an example:
$write_secs = (stat($file))[9];
- print "file $file updated at ", scalar(localtime($file)), "\n";
+ printf "file %s updated at %s\n", $file,
+ scalar localtime($write_secs);
If you prefer something more legible, use the File::stat module
(part of the standard distribution in version 5.004 and later):
+ # error checking left as an exercise for reader.
use File::stat;
use Time::localtime;
$date_string = ctime(stat($file)->mtime);
print "file $file updated at $date_string\n";
-Error checking is left as an exercise for the reader.
+The POSIX::strftime() approach has the benefit of being,
+in theory, independent of the current locale. See L<perllocale>
+for details.
=head2 How do I set a file's timestamp in perl?
($atime, $mtime) = (stat($timestamp))[8,9];
utime $atime, $mtime, @ARGV;
-Error checking is left as an exercise for the reader.
+Error checking is, as usual, left as an exercise for the reader.
Note that utime() currently doesn't work correctly with Win95/NT
ports. A bug has been reported. Check it carefully before using
open (FH, "| tee file1 file2 file3");
-Otherwise you'll have to write your own multiplexing print function --
-or your own tee program -- or use Tom Christiansen's, at
-http://www.perl.com/CPAN/authors/id/TOMC/scripts/tct.gz, which is
-written in Perl.
+Or even:
+
+ # make STDOUT go to three files, plus original STDOUT
+ open (STDOUT, "| tee file1 file2 file3") or die "Teeing off: $!\n";
+ print "whatever\n" or die "Writing: $!\n";
+ close(STDOUT) or die "Closing: $!\n";
+
+Otherwise you'll have to write your own multiplexing print
+function -- or your own tee program -- or use Tom Christiansen's,
+at http://www.perl.com/CPAN/authors/id/TOMC/scripts/tct.gz, which is
+written in Perl and offers much greater functionality
+than the stock version.
+
+=head2 How can I read in an entire file all at once?
+
+The customary Perl approach for processing all the lines in a file is to
+do so one line at a time:
+
+ open (INPUT, $file) || die "can't open $file: $!";
+ while (<INPUT>) {
+ chomp;
+ # do something with $_
+ }
+ close(INPUT) || die "can't close $file: $!";
-In theory a IO::Tee class could be written, but to date we haven't
-seen such.
+This is tremendously more efficient than reading the entire file into
+memory as an array of lines and then processing it one element at a time,
+which is often -- if not almost always -- the wrong approach. Whenever
+you see someone do this:
+
+ @lines = <INPUT>;
+
+You should think long and hard about why you need everything loaded
+at once. It's just not a scalable solution. You might also find it
+more fun to use the standard DB_File module's $DB_RECNO bindings,
+which allow you to tie an array to a file so that accessing an element
+the array actually accesses the corresponding line in the file.
+
+On very rare occasion, you may have an algorithm that demands that
+the entire file be in memory at once as one scalar. The simplest solution
+to that is:
+
+ $var = `cat $file`;
+
+Being in scalar context, you get the whole thing. In list context,
+you'd get a list of all the lines:
+
+ @lines = `cat $file`;
+
+This tiny but expedient solution is neat, clean, and portable to
+all systems on which decent tools have been installed. For those
+who prefer not to use the toolbox, you can of course read the file
+manually, although this makes for more complicated code.
+
+ {
+ local(*INPUT, $/);
+ open (INPUT, $file) || die "can't open $file: $!";
+ $var = <INPUT>;
+ }
+
+That temporarily undefs your record separator, and will automatically
+close the file at block exit. If the file is already open, just use this:
+
+ $var = do { local $/; <INPUT> };
=head2 How can I read in a file by paragraphs?
-Use the C<$
\> variable (see L<perlvar> for details). You can either
+Use the C<$
/> variable (see L<perlvar> for details). You can either
set it to C<""> to eliminate empty paragraphs (C<"abc\n\n\n\ndef">,
for instance, gets treated as two paragraphs and not three), or
C<"\n\n"> to accept empty paragraphs.
+Note that a blank line must have no blanks in it. Thus C<"fred\n
+\nstuff\n\n"> is one paragraph, but C<"fred\n\nstuff\n\n"> is two.
+
=head2 How can I read a single character from a file? From the keyboard?
You can use the builtin C<getc()> function for most filehandles, but
the Term::ReadKey module from CPAN, or use the sample code in
L<perlfunc/getc>.
-If your system supports POSIX, you can use the following code, which
-you'll note turns off echo processing as well.
+If your system supports the portable operating system programming
+interface (POSIX), you can use the following code, which you'll note
+turns off echo processing as well.
#!/usr/bin/perl -w
use strict;
END { cooked() }
-The Term::ReadKey module from CPAN may be easier to use:
+The Term::ReadKey module from CPAN may be easier to use. Recent version
+include also support for non-portable systems as well.
use Term::ReadKey;
open(TTY, "</dev/tty");
printf "\nYou said %s, char number %03d\n",
$key, ord $key;
-For DOS systems, Dan Carson <dbc@tc.fluke.COM> reports the following:
+For legacy DOS systems, Dan Carson <dbc@tc.fluke.COM> reports the following:
To put the PC in "raw" mode, use ioctl with some magic numbers gleaned
from msdos.c (Perl source file) and Ralf Brown's interrupt list (comes
This is all trial and error I did a long time ago, I hope I'm reading the
file that worked.
-=head2 How can I tell if there's a character waiting on a filehandle?
+=head2 How can I tell whether there's a character waiting on a filehandle?
+
+The very first thing you should do is look into getting the Term::ReadKey
+extension from CPAN. As we mentioned earlier, it now even has limited
+support for non-portable (read: not open systems, closed, proprietary,
+not POSIX, not Unix, etc) systems.
-You should check out the Frequently Asked Questions list in
+You should also check out the Frequently Asked Questions list in
comp.unix.* for things like this: the answer is essentially the same.
It's very system dependent. Here's one solution that works on BSD
systems:
return $nfd = select($rin,undef,undef,0);
}
-You should look into getting the Term::ReadKey extension from CPAN.
+If you want to find out how many characters are waiting, there's
+also the FIONREAD ioctl call to be looked at. The I<h2ph> tool that
+comes with Perl tries to convert C include files to Perl code, which
+can be C<require>d. FIONREAD ends up defined as a function in the
+I<sys/ioctl.ph> file:
-=head2 How do I open a file without blocking?
+ require 'sys/ioctl.ph';
-You need to use the O_NDELAY or O_NONBLOCK flag from the Fcntl module
-in conjunction with sysopen():
+ $size = pack("L", 0);
+ ioctl(FH, FIONREAD(), $size) or die "Couldn't call ioctl: $!\n";
+ $size = unpack("L", $size);
- use Fcntl;
- sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
- or die "can't open /tmp/somefile: $!":
+If I<h2ph> wasn't installed or doesn't work for you, you can
+I<grep> the include files by hand:
-=head2 How do I create a file only if it doesn't exist?
+ % grep FIONREAD /usr/include/*/*
+ /usr/include/asm/ioctls.h:#define FIONREAD 0x541B
-You need to use the O_CREAT and O_EXCL flags from the Fcntl module in
-conjunction with sysopen():
+Or write a small C program using the editor of champions:
- use Fcntl;
- sysopen(FH, "/tmp/somefile", O_WRONLY|O_EXCL|O_CREAT, 0644)
- or die "can't open /tmp/somefile: $!":
+ % cat > fionread.c
+ #include <sys/ioctl.h>
+ main() {
+ printf("%#08x\n", FIONREAD);
+ }
+ ^D
+ % cc -o fionread fionread.c
+ % ./fionread
+ 0x4004667f
-Be warned that neither creation nor deletion of files is guaranteed to
-be an atomic operation over NFS. That is, two processes might both
-successful create or unlink the same file!
+And then hard-code it, leaving porting as an exercise to your successor.
+
+ $FIONREAD = 0x4004667f; # XXX: opsys dependent
+
+ $size = pack("L", 0);
+ ioctl(FH, $FIONREAD, $size) or die "Couldn't call ioctl: $!\n";
+ $size = unpack("L", $size);
+
+FIONREAD requires a filehandle connected to a stream, meaning sockets,
+pipes, and tty devices work, but I<not> files.
=head2 How do I do a C<tail -f> in perl?
filehandle. The method: read until end of file, clearerr(), read some
more. Lather, rinse, repeat.
+There's also a File::Tail module from CPAN.
+
=head2 How do I dup() a filehandle in Perl?
If you check L<perlfunc/open>, you'll see that several of the ways
$fd = $ENV{MHCONTEXTFD};
open(MHCONTEXT, "<&=$fd"); # like fdopen(3S)
-Error checking has been left as an exercise for the reader.
+Note that "<&STDIN" makes a copy, but "<&=STDIN" make
+an alias. That means if you close an aliased handle, all
+aliases become inaccessible. This is not true with
+a copied one.
+
+Error checking, as always, has been left as an exercise for the reader.
=head2 How do I close a file descriptor by number?
$rc = syscall(&SYS_close, $fd + 0); # must force numeric
die "can't sysclose $fd: $!" unless $rc == -1;
+Or just use the fdopen(3S) feature of open():
+
+ {
+ local *F;
+ open F, "<&=$fd" or die "Cannot reopen fd=$fd: $!";
+ close F;
+ }
+
=head2 Why can't I use "C:\temp\foo" in DOS paths? What doesn't `C:\temp\foo.exe` work?
Whoops! You just put a tab and a formfeed into that filename!
backslash is an escape character. The full list of these is in
L<perlop/Quote and Quote-like Operators>. Unsurprisingly, you don't
have a file called "c:(tab)emp(formfeed)oo" or
-"c:(tab)emp(formfeed)oo.exe" on your DOS filesystem.
+"c:(tab)emp(formfeed)oo.exe" on your legacy DOS filesystem.
Either single-quote your strings, or (preferably) use forward slashes.
Since all DOS and Windows versions since something like MS-DOS 2.0 or so
have treated C</> and C<\> the same in a path, you might as well use the
one that doesn't clash with Perl -- or the POSIX shell, ANSI C and C++,
-awk, Tcl, Java, or Python, just to mention a few.
+awk, Tcl, Java, or Python, just to mention a few. POSIX paths
+are more portable, too.
=head2 Why doesn't glob("*.*") get all the files?
Because even on non-Unix ports, Perl's glob function follows standard
Unix globbing semantics. You'll need C<glob("*")> to get all (non-hidden)
-files.
+files. This makes glob() portable even to legacy systems. Your
+port may include proprietary globbing functions as well. Check its
+documentation for details.
=head2 Why does Perl let me delete read-only files? Why does C<-i> clobber protected files? Isn't this a bug in Perl?
This is elaborately and painstakingly described in the "Far More Than
-You Every Wanted To Know" in
+You Ever Wanted To Know" in
http://www.perl.com/CPAN/doc/FMTEYEWTK/file-dir-perms .
The executive summary: learn how your filesystem works. The
rand($.) < 1 && ($line = $_) while <>;
This has a significant advantage in space over reading the whole
-file in.
+file in. A simple proof by induction is available upon
+request if you doubt its correctness.
+
+=head2 Why do I get weird spaces when I print an array of lines?
+
+Saying
+
+ print "@lines\n";
+
+joins together the elements of C<@lines> with a space between them.
+If C<@lines> were C<("little", "fluffy", "clouds")> then the above
+statement would print:
+
+ little fluffy clouds
+
+but if each element of C<@lines> was a line of text, ending a newline
+character C<("little\n", "fluffy\n", "clouds\n")> then it would print:
+
+ little
+ fluffy
+ clouds
+
+If your array contains lines, just print them:
+
+ print @lines;
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997 Tom Christiansen and Nathan Torkington.
-All rights reserved. See L<perlfaq> for distribution information.
+Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
+All rights reserved.
+
+When included as an integrated part of the Standard Distribution
+of Perl or of its documentation (printed or otherwise), this works is
+covered under Perl's Artistic License. For separate distributions of
+all or part of this FAQ outside of that, see L<perlfaq>.
+
+Irrespective of its distribution, all code examples here are in the public
+domain. You are permitted and encouraged to use this code and any
+derivatives thereof in your own programs for fun or for profit as you
+see fit. A simple comment in the code giving credit to the FAQ would
+be courteous but is not required.
https://perl5.git.perl.org / perl5.git / blobdiff